AUSTRIAN JOURNAL of EARTH SCIENCES [MITTEILUNGEN der ÖSTERREICHISCHEN GEOLOGISCHEN GESELLSCHAFT] an INTERNATIONAL JOURNAL of the AUSTRIAN GEOLOGICAL SOCIETY volume 98 2005 Gerd RANTITSCH & Barbara RUSSEGGER: Organic maturation within the Central Northern Calcareous Alps (Eastern Alps)________________________________ www.univie.ac.at/ajes EDITING: Grasemann Bernhard, Wagreich Michael PUBLISCHER: Österreichische Geologische Gesellschaft Rasumofskygasse 23, A-1031 Wien TYPESETTER: Irnberger Norbert, www.irnberger.net Copy-Shop Urban, Bahnstraße 26a, 2130 Mistelbach PRINTER: Holzhausen Druck & Medien GmbH Holzhausenplatz 1, 1140 Wien ISSN 0251-7493 Austrian Journal of Earth Sciences Volume 98 Vienna 2005 Organic maturation within the Central Northern Calca- reous Alps (Eastern Alps) Gerd RANTITSCH1) & Barbara RUSSEGGER2) KEYWORDS very low-grade metamorphism Northern Calcareous Alps 1) Department of Applied Geosciences and Geophysics, Montanuniversität Leoben, vitrinite reflectance Austria, A-8700 Leoben, Austria, Email: [email protected] thermal modeling Jurassic orogeny 2) Library of the University of Graz, A-8010 Graz, Austria Eastern Alps Abstract Organic maturation within the central Northern Calcareous Alps (Eastern Alps) has been investigated using vitrinite reflectance data from Upper Permian to Upper Cretaceous strata of the Stauffen-Höllengebirge Nappe, Dachstein Nappe, Tennengebirge Block and Lammer Unit. Within the Lammer Unit a metamorphic break is observed between the Strubberg Formation and the tectonically overlying Schwarzenberg Complex and Losegg-Hofpürgl slice. Vitrinite reflectance within the Dachstein Nappe and the Stauffen-Höllengebirge Nappe resembles the values within the Schwarzenberg Complex. The thermal overprint of the Tennengebirge Block and the Strubberg Formation is explained by metamorphism due to Late Jurassic imbrication of fault-bounded blocks. A numerical heat flow model explains the thermal overprint of the Dachstein Nappe by sedimentary burial during Mesozoic to Cenozoic times._________________________ Im Zentralteil der Nördlichen Kalkalpen wurde der Grad der organischen Metamorphose durch die Messung der Vitrinitreflexion in oberpermischen bis oberkretazischen Sedimenten der Stauffen-Höllengebirge Decke, der Dachstein Decke, des Tennengebirge Blocks und der Lammer Einheit erfasst. Der tektonische Kontakt zwischen der Lammer Einheit und den tektonisch aufliegenden Komplexen des Schwarzenbergs und der Losegg-Hofpürgl Schuppe ist durch einen Metamorphosesprung gekennzeichnet. Die Vitrinitreflexion innerhalb der Dachstein- und Stauffen-Höllengebirge Decke entspricht den Werten innerhalb des Schwarzenberg Komplexes. Die thermische Überprägung des Tennengebirge Blocks und der Strubberg Formation wird durch die metamorphe Überprägung während der spätjuras- sischen Imbrikation von störungsbegrenzten Blöcken erklärt. Ein numerisches Wärmeflussmodell erklärt die thermische Überprägung der Dachstein Decke durch die mesozoische bis känozoische sedimentäre Auflast der Schichtfolge.___________________________ 1. Introduction The Northern Calcareous Alps (NCA) are an elongated fold- closer insight to thermal processes in higher crustal levels. and-thrust-belt (Linzer et al. 1995) within the Eastern Alps (Fig. However, the central segment of the NCA, the key area of the still 1). The classical tectonic subdivision describes a lower Bavaric, ongoing debate about the internal structure of the NCA an intermediate Tirolic and an upper Juvavic nappe group (Tollmann, 1976 vs. Neubauer, 1994 and Schweigl and (Tollmann, 1976; Mandl, 2000). Whereas the Bavaric and the Neubauer, 1997 vs. Frisch and Gawlick, 2003), is only sparsely Tirolic units involve sediments of a Triassic to Jurassic carbonate covered by organic maturity data. In this contribution, we present platform facing the western Tethyan Ocean, the Juvavic units therefore vitrinite reflectance data to complete the pattern of derived from the platform rim and outer shelf area (Hallstatt organic metamorphism and to give an insight in the thermal Zone). Based on stratigraphic, structural, metamorphic and structure of this section. geochronological data from the central NCA, Frisch and Gawlick (2003) proposed an alternative nappe concept in which the Ju- 2. Geological setting vavic Dachstein Nappe is a subunit of the Tirolic nappe group The Late Triassic facial distribution of the NCA describes a and deeper shelf sediments of the former Hallstatt Zone are carbonate platform (Dachstein platform) faced in the south by the parts of a flysch succession (Hallstatt mélange).____________ outer shelf and shelf edge realm of the Hallstatt Zone (Haas et al., Because the thermal structure of fold-and–thrust-belts is us- 1995, Gawlick et al., 1999; Mandl, 2000). This facies belt was ually attributed to thrust-related processes like burial by thrust destroyed during the Late Jurassic closure of the adjacent sheets or shear heating along thrust zones (e.g. Warr et al., Meliata Ocean (Neubauer, 1994; Neubauer et al., 2000; Mandl, 1996), detailed metamorphic studies can contribute significantly 2000; Frisch and Gawlick, 2003). Late Jurassic to Eocene to controversies about opposing nappe concepts. It was de- thrusting (Mandl, 2000; Frisch and Gawlick, 2003) and Early to monstrated in numerous studies that the use of organic meta- Middle Miocene block fragmentation (Linzer et al., 1995; Frisch morphic parameters resulted in the construction of the most et al., 1998; Frisch and Gawlick, 2003) determined the present detailed and meaningful metamorphic maps. Moreover, the tectonic structure of the NCA. sensitive and irreversible response of organic matter to subtle In the traditional model (Tollmann, 1976; Mandl, 2000), Late temperature variations and the well-known kinetics of organic Jurassic to Cretaceous north-directed thrusting displaced the maturation allow the calibration of thermal models which give a deeper shelf and platform sediments of the Hallstatt Zone as a 68 Gerd RANTITSCH & Barbara RUSSEGGER Juvavic Nappe Complex (involving the Dachstein Nappe, Fig. 2) bituminous Carnian (Lunz) coals, implying maximum burial upon the Tirolic Nappe Complex (involving the Stauffen-Höllen- temperatures of c. 170° C (Sachsenhofer, 1987). gebirge Nappe, Fig. 2). Jurassic basins of the Tirolic Nappe According to Frisch and Gawlick (2003) the Late Jurassic Complex containing gravity transported slices (olistoliths) of the imbrication of the accretionary wedge metamorphosed the Hallstatt Zone were overthrusted by the Juvavic Nappe Complex. olistoliths of the Hallstatt Zone. Conodont Alteration Indices The contrary model by Frisch and Gawlick (2003) describes a (ranging from CAI 1.0 to CAI 7.0) within carbonate olistoliths of Callovian to Oxfordian accretionary wedge which was com- the Hallstatt Zone (Gawlick and Königshof, 1993; Gawlick et al., pletely destroyed during the Late Jurassic. The debris of this 1994; Gawlick and Höpfer, 1996) support this model. According wedge was transported towards the north into a radiolaritic flysch to data from the entire NCA, Spötl and Hasenhüttl (1998) basin (Lammer Basin) which subsided on top of the Dachstein concluded that the imbrication is responsible for the variation of carbonate platform. Progressive north-directed thrusting vitrinite reflectance and illite crystallinity in the Upper Permian separated the Dachstein carbonate platform into a lower- and an Haselgebirge. From calcite-dolomite thermometry Gawlick and upper-Tirolic nappe. The Stauffen-Höllengebirge Nappe is Höpfer (1996) supposed maximum burial temperatures of 350° C interpreted as a segment of the lower-Tirolic Nappe whereas the to 490° C at minimum pressures of 6-8 kbar during this event. A Dachstein Nappe is interpreted as a segment of the upper-Tirolic large segment of the wedge (Tennengebirge block of the Ultra- Nappe. During Oxfordian to Tithonian times radiolaritic basins Tirolic nappe sensu Frisch and Gawlick, 2003) was deeply buried developed at the frontal ramp of the upper Tirolic nappe and metamorphosed (Frisch and Gawlick, 2003). Kralik and (Tauglboden Basin) and out-of-sequence upon the Lammer Schramm (1994) explained the lower thermal overprint in the Basin (Sillenkopf Basin). During the Eocene period, out-of- western segment of the Stauffen-Höllengebirge Nappe by a sequence thrusting transported the Dachstein Nappe upon younger metamorphic event which was induced by thrusting sediments of the Hallstatt Zone (Frisch and Gawlick, 2003). combined with fluid circulation. Geochronological data constrain In the western and central part of the NCA there is a general the age of metamorphism between 130 and 140 Ma (Kralik et al., north to south and east to west increase in the metamorphic 1987; Kralik and Schramm, 1994). overprint (Kralik et al., 1987; Krumm et al., 1988; Petschick, 1989; Progressive thrusting was followed by a phase of uplift and Kürmann, 1993; Ferreiro Mählmann, 1994). In this segment, erosion during the Early Cretaceous before the Gosau basins Petschick (1989) and Ferreiro Mählmann and Petschick (1995) (Figs. 1, 2) subsided during Turonian to Eocene times on top of proposed a (pre-tectonic) Permian to Early Cretaceous dias- the nappe stack (Wagreich and Faupl, 1994; Wagreich, 1995, tathermal metamorphism with burial temperatures up to 300° C, 2001; Wagreich and Decker, 2001). a syn-tectonic thermal event during Early Cretaceous to Turonian The Lammer Unit (Tollmann, 1976; Fig. 2) is the key area for the times with burial temperatures of